Pro-Russian separatists took a Buk system last month, but it was not reported whether the theft was from the Russian or Ukrainian militaries.

The United States Federal Aviation Administration warned US aircraft in April not to fly over part of Crimea, the disputed area between Russia and Ukraine.

The no-go zone did not include the area where Malaysia Airlines flight MH 17 crashed this morning , about 322km away.

It was not unusual to fly at an increased height over countries experiencing civil conflict that was "not a fully blown war", Aviation Advocacy founder and chief executive officer Andrew Charlton told Radio New Zealand.

Air New Zealand had flown over conflict zones such as Afghanistan and Iraq en route to London "for many years", Charlton said.

Airlines made decisions about where to fly based on factors such as the wind and other flight plan, he said.

It was only after MH17 crashed in Ukraine this morning that other airlines tweeted to say they had changed their policies and would not fly over the country, he said.

If an aircraft was travelling at an altitude of 10,000 metres, only "remarkably sophisticated weaponry" could bring it down, he said.

The Buk ground-to-air missile launchers possessed by Ukrainian government and pro-Russia separatist forces had a range that could hit an aircraft at this height, but it would require "fine marksmanship" to hit a plane travelling at 900kmh and would be a deliberate act, he said.

That the commercial aircraft had been mistaken for a military one could not be ruled out, he said.

There was no way for the plane to signal its status, as civilian and military aviators operated on different systems, he said.

Airlines had already filed their flight plans for the day, and would have to contact and divert their flights that were underway, he said.

The Ukraine was under a "very common flight path" for flying from Europe to Southeast Asia, and aircraft would be diverted south in the coming days, which may increase flight time and fuel useage, depending on the wind, he said.